

His origins are revealed through a series of flashbacks. It is theorized that the strength of his powers is inversely proportional to his desire to live, that is to say, his powers are at his strongest the least he values his own life, but weaken considerably the more he clings to life. Although emotionally detached, blunt and indifferent of others at first, he begins to slowly grow a soft spot for the rest of the club, specially Hanasaki whom he wants to keep his promise to kill him one day. He's reluctantly recruited by Hanasaki as part of the Boy Detectives Club after Hanasaki takes an interest on him and after realizing that somehow, getting involved in their cases injures him where everything else fails.

A mysterious boy with supernatural abilities, including immortality who seeks to die but is unable to do so. Characters Yoshio Kobayashi ( 小林 芳雄, Kobayashi Yoshio) Voiced by: Daiki Yamashita (Japanese) Austin Tindle (English) One of the main characters. The encounter between the two eventually leads them to the connection between the criminal nicknamed " Fiend with Twenty Faces" (Kaijin Nijuu Menso) and Kogoro Akechi. Although he does not have much use for people, he gradually begins to acknowledge the group as he spends more time with them while solving cases. The apathetic Yoshio accepts this deal begrudgingly, unaware of how different his life will become. After seeing his abilities in action, Kensuke offers Yoshio a deal: join the Boy Detectives' Club and help them solve cases, and in exchange he will find a way to help Yoshio die. Kobayashi, who has an undying body because of an "unidentifiable fog," wishes his own death and refuses contact with others.

One of their junior members, Kensuke Hanasaki, is out solving a case one day when he happens upon Yoshio Kobayashi. Together, this group takes on cases both great and small. The story is set in 203X, and the «Shounen Tantei-Dan» ( 少年探偵団, Shōnen Tantei-Dan, «Boy Detectives Club») have been assembled under the mysterious detective Kogoro Akechi.
